Micromachined Silicon Ultrasonic Longitudinal Mode Actuators: Theory and Applications to Surgery, Pumping, and Atomization

A silicon-based high-intensity ultrasound actuator technology is developed. It is shown that silicon's high maximum ultrasonic particle velocity, high thermal conductivity, and low internal loss, make it an ideal resonator material for high-intensity ultrasonic actuators. Furthermore, it is expected that silicon micromachined batch-fabrication and sensor integration will permit realizing low-cost, possibly disposable, intelligent ultrasonic actuators. Comparison of the silicon-based to the conventional titanium-based actuators is made.
A coupled-acoustic waveguide model of the piezoelectric/silicon adhesively bonded composite resonator is presented. The analysis, when applied to a piezoelectric PZT (lead zirconate titanate) and silicon laminate, shows that there is an optimum choice of PZT and silicon plate dimensions that maximizes the output power of the resonant actuator. The model is also used to develop a method of measuring the complex shear modulus of the bonding adhesive.
Silicon ultrasonic horns (linear, catenary, exponential, and stepped) were designed and microfabricated to concentrate ultrasonic energy and magnify displacements. Horns with etched V-grooves were bonded together to form silicon ultrasonic needles. One of these needles was able to produce 100 umpp tip displacements at 72 kHz with a drive of 70-80 Vpp. The corresponding tip velocity (23 m/s) is almost twice the typical maximum tip velocity (12 m/s) of the conventional titanium-based actuators.
The three mechanisms of ultrasonic surgery- direct cutting, cavitation, and micro- strearning- are discussed. A model based on direct cutting is used to estimate the power required for surgery. Experiments with cutting human cataractous eye lens and tissue simulants are described.
Experimental evidence and theory of shallow-channel mode of ultrasonic liquid pumping, based on acoustic streaming inside the silicon ultrasonic needle, is presented. A new mode of pumping, due to vortex formation near the vibrating tip of the ultrasonic needle was discovered experimentally. The observed flow patterns and a physical mechanism for this vortex-pumping mode are presented.
A theory of ultrasonic atomization based on capillary wave ejection and cavitation is presented. The silicon ultrasonic needle was used to atomize water into droplets with diameters ranging from 10-40 um. Isopropanol was also atomized, and combusted at flow rates of 2.4 ml/min.
Publication date: 
December 31, 1996
Publication type: 
Ph.D. Dissertation
Lal, A. (1996). Micromachined Silicon Ultrasonic Longitudinal Mode Actuators: Theory and Applications to Surgery, Pumping and Atomization. United States: University of California, Berkeley.

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